Most of these use homonyms that don't appear in their English counterparts. I suspect you know that already: one doesn’t come up with all these randomely. Just to satisfy your curiosity: “When I heard the siren, I saw a mermaid” → «Quand j’ai entendu la sirène, j’ai vu une sirène» (nice play on words, actually, that seemingly turns a sound into something visible). In order to avoid this, one may choose «Quand j’ai entendu l’alarme, j’ai vu une sirène».
– Montée de laitNov 27 '18 at 14:02

How do you say "L'un cherche la puissance, l'autre cherche le pouvoir" in English? There are lots of words that have synonyms in one language but not in the other. Asking "how do you say that?" is not really a question, you know we can't.
– Teleporting GoatNov 29 '18 at 10:14

1 Answer
1

Short explanation: each sentence contains two words that translate as one unique French word. These homonyms can be differenciated with the sentence context.

With my headlights on, I could see the lighthouse.

Avec mes phares allumés, je pouvais voir le phare.

The “phares” most people turn on are the headlights. The “phare” is probably not a car's headlight because it is singular.

When I heard the siren, I saw a mermaid.

Quand j'entendis la sirène d'alarme, je vis une sirène.

Both “sirènes” can be heard, but only one can be seen. Then, we do not have to precise the second “sirène” is a mythologic critture. We still have to add details on the first one in order to show it is about a siren, not a mermaid.

I want an office with a desk in it.

Je veux un bureau avec un bureau à l'intérieur

“Je veux un bureau” alone needs a context in order to understand it. In this sentence, it is almost certain I want a room with furniture in it, and not the opposite!

A walnut is a nut.

Une noix est un fruit à enveloppe ligneuse.

Using “noix” instead of “arachide” “fruit à enveloppe ligneuse”1 is an “abus de langage”.

My fireplace has a chimney.

French does not have an equivalent to "fireplace". It can be translated to “cheminée, foyer, âtre” these last two being parts of a chimney.

I use my tongue to speak a language.

When we use “parler” with “langue” (language), it is for the quantifier between them: “Je parle une, deux, cinq langues.” However, we just talk. If we absolutely need a translation: